For climatologists, the arctic tundra was a straightforward example of a carbon sink: The vast, treeless, permafrosted plain took in more carbon from the atmosphere through photosynthesis than it released through decay and respiration.

Members of the Gas Flux Study, part of NSF's Arctic System Science Program, pulled the plug on the "carbon sink" image last December at the American Geophysical Union meeting in San Francisco. "Some years the tundra is adding more carbon to the atmosphere than it removes, although the total amount of carbon released to the atmosphere is still quite small," says biologist George Kling of the University of Michigan.

The arctic tundra and its carbon will play an important role in global change. Carbon dioxide is a greenhouse gas, and increased amounts of it in the atmosphere may lead to global warming. The arctic tundra currently has enough carbon locked up in its system to increase the carbon dioxide in the atmosphere by 33 percent.

In their three-year study in Alaska's Kuparuk River Basin, the team found that "arctic plants are still taking in carbon dioxide from the atmosphere during photosynthesis," Kling explains, "but instead of much of that carbon remaining locked up in the soil, more of it is being respired back into the atmosphere."

Equally critical for future predictions, the team discovered a measurement problem. "Five or six years ago, all of the estimates were made only on the terrestrial side," says Kling. These estimates consistently ignored the tundra's streams and lakes and therefore underestimated the amount of carbon released.

The team found that every square meter of tundra released an average of five grams of carbon per year through water. About half of it is carried off as dissolved carbon, says Kling, while the other half goes directly back into the atmosphere.

"We have known for some time that arctic lakes and streams are supersaturated with carbon dioxide and methane, and that this excess gas diffuses into the atmosphere," Kling says. "What we didn't know is just how much carbon is entering the atmosphere through contact with surface waters."

The team, which includes John Hobbie and Ed Rastetter of the Ecosystems Center in Woods Hole, Mass., as well as Terry Chapin of the University of California at Berkeley and Walter Oechel of San Diego State University, used the computer models developed at Woods Hole to quantify the amount of carbon flux from the arctic tundra into the global ecosystem.

However, Kling notes, these numbers are only for the current climate. If the arctic gets warmer, as predicted, all bets are off.

A warmer arctic adds new variables to the equation. It may mean more carbon and more methane, or just more carbon. It might also mean more nitrogen, because plants will decay faster. Since most of the tundra plants are limited by the amount of nitrogen in the soil, more nitrogen would mean more plant growth.

"We know that since the last ice age there have been lots of changes to the vegetation. When it was warmer, there were trees on Alaska's North Coast," Kling says, marveling at this concept. "What we don't know is what exactly controlled this shift."